The present invention relates generally to motors, and more particularly to fast discharge of a ring motor field.
High-power synchronous alternating current (AC) motors are used in a number of industrial applications. In mining, for example, high-power synchronous AC motors are used for gearless draglines (with a capacity of ˜10,000 horsepower), gearless conveyor drives, and gearless mill drives. High-power synchronous AC motors are also deployed in other systems, such as wind turbines.
A salient pole AC synchronous motor includes a set of polyphase distributed windings called the stator and a rotor with a field winding. The stator windings are fed with AC. Field circuits or excitation systems for the field winding can be energized using a DC source. The DC source can be obtained, for example, from a three-phase AC power supply using an active front end (AFE) rectifier that charges a DC link and then using a DC chopper to get the necessary field excitation. The same DC link can also be used to feed an inverter circuit that provides the AC source for the stator windings. The DC link, the inverter, and the input to the stator are prone to faults.
Large drive currents are used in operation. Since the field winding of a high-power AC synchronous motor is inductive, substantial electromagnetic energy is stored in the field winding. In the event of a DC link short circuit fault or a stator side short circuit fault, the field circuit needs to be de-energized, and the stator circuit needs to be isolated from the AC source. Typically, field de-energization is achieved through a field crowbar circuit. The discharge time is long, and high speed circuit breakers are needed to disconnect the stator windings from the AC source. The field energy is dissipated as waste heat.